A thin metal sheet is used as material for composing members for a vehicle body, other equipment or apparatus. In order to give the required strength to a demarcated and predetermined region in the article produced of the thin sheet, the whole of the predetermined region is heated to a temperature equal to or more than a target temperature for hardening. As apparatuses with which the heating is performed by an induction heating method using a high-frequency current, apparatuses in patent documents 1 and 2 mentioned below are known.
In an apparatus in patent document 1, an inductive portion of a heating inductor to which a high-frequency current is applied is movable in relationship to an article made of a thin sheet, and the inductive portion is moved in relation to the article made of the thin sheet to thereby heat a region of the article made of the thin sheet whereto the inductive portion is moved, by an inductive eddy-current. According to this apparatus, though a heating temperature can be adjusted in accordance with a setup of a moving speed, processes to move the inductive portion for every heating work of respective articles are required, and, therefore, it takes time to process only one piece of the article and a lot of articles can be hardly processed in a short time effectively.
In contrast with the above, an inductive portion of a heating inductor in the apparatus of patent document 2 corresponds to the whole of the region to be heated in an article made of a thin sheet. Therefore, according to the apparatus of patent document 2, a bulk heating can be realized, whereby the whole of the region to be heated can be heated simultaneously only by applying a high-frequency current to a heating inductor. The apparatus can treat respective articles in a short time, as compared with the apparatus in patent document 1 and, as a result, the working efficiency can be improved.
Patent document 1: Japanese Patent Application Laid-open No. Hei 10-17933 (paragraph number 0042, FIG. 4)
Patent document 2: Japanese Patent Application Laid-open No. 2000-256733 (paragraph number 0045, FIG. 1)